The present invention relates to a fatigue damage detection sensor for structural materials and mounting method thereof, and in particular, to a fatigue damage detection sensor for structural materials that senses the history of stress and the level of fatigue damage caused to various structural materials or members (base materials) such as steel materials which are used to build structures such as bridges, iron towers, and other buildings as well as machine structures such as construction machines, to mounting method thereof capable of accurately and reliably mounting the fatigue damage detection sensor so as to provide a predetermined sensitivity.
Conventional structural materials may be fatigue-damaged under a working load, and it is essential to periodically inspect these materials to maintain safety.
These structural materials, however, not only have a large number of inspection items to be constantly checked but also require expertise and experience for visual inspections, scaffolding for inspections that is required due to a possible danger involved in these operations, a large amount of time for inspections, and the continuance of inspections over a long time period. Thus, there is a need to accurately and efficiently carry out determination of the fatigue damage condition of these materials and sensing of abnormality.
In such inspections or maintenance and management, sensing of fatigue damage is divided into three historical stages including a first stage in which no crack has not occurred despite an accumulated fatigue, a second stage in which damage has occurred as a crack due to the fatigue, and a third stage in which the fatigue has occurred to propagate the crack.
In each of these stages, for example, the first stage requires the level of fatigue accumulation to be determined to sense the possibility of the occurrence of a fatigue crack, the second stage requires a generated crack to be sensed, and the third stage requires the current propagation of a crack to be determined to predict future propagation and a point of time at which the material may be destroyed.
However, the history of stress that may cause a crack was conventionally determined by checking the design and measuring stress over a specified time period. This determination, however, has been inaccurate and has required a large amount of time and costs. That is, there has not been a practical sensor for sensing not only the occurrence but also the propagation of a crack or a method for reliably mounting a detection sensor with a predetermined sensing accuracy and low costs.
The present invention has been achieved in view of these problems, and its object is to provide a fatigue damage detection sensor for structural materials and mounting method thereof wherein the fatigue damage detection sensor can be stably reliably mounted on the material so as to provide a good sensing performance.
Furthermore, another object of this invention is to provide a fatigue damage detection sensor for structural materials and mounting method thereof wherein the sensor can be mounted with a predetermined accuracy so as to sense the occurrence and propagation of a crack appropriately.
Yet another object of this invention is to provide a fatigue damage detection sensor for structural materials and mounting method thereof wherein the sensor allows a crack to be stably generated and propagated on behalf of the structural material without being affected by external environments such as the temperature and humidity so as to predict the level of fatigue damage caused to the structural material based on the rate or amount of propagation.
That is, the present invention focuses on a predetermined relationship between the length of the fatigue damage detection sensor and a required sensitivity or sensing accuracy (crack propagation rate) and attempts to increase the mean stress beforehand by applying this relationship, forming a fatigue pre-crack in the fatigue damage detection sensor beforehand, leveling or relieving a residual stress, and applying an initial stress prior to mounting. A first invention is a fatigue damage detection sensor for structural materials for sensing the stress history of a structural material that is subjected to a working load as well as the level of fatigue damage caused to the material, characterized in that the sensor comprises a detection sensor body configured by a rectangular panel-shaped member that can be mounted on a surface of the structural material and that has a specified width and a predetermined length, the detection sensor body including a notched portion on one or both length-wise sides thereof or inside the plate-shaped member; and a crack detection means located in front of the tip of the notched portion and provided in the sensor body and in that when the length between a pair of fixing portions for fixing to the surface of the structural material, both ends of the sensor body sandwiching the notched portion is defined as 2H, the length of a crack that can propagate from the tip of the notched portion is defined as (a), the number of times that a working load acts on the material is defined as N, and the crack propagation rate is defined as da/dN, the length 2H between the fixing portions is set so as to obtain a required sensitivity with which da/dN is proportional to H1.5.
The above described notched portion has a fatigue pre-crack formed at its tip, and a residual stress is relieved from the fatigue pre-crack to increase the mean stress before the sensor body is mounted on the structural material.
A second invention is a method for mounting a fatigue damage detection sensor for structural materials for sensing the level of fatigue damage caused to a structural material that is subjected to an external stress, characterized by comprising the notched portion formation step of forming a notched portion on one or both length-wise sides of a sensor body configured by a rectangular panel-shaped member that can be mounted on the surface of the structural material and that has a specified width and a predetermined length or forming the notched portion inside the panel-shaped member; the fatigue pre-crack formation step of forming a fatigue pre-crack in the notched portion, the residual-stress relief step of relieving the residual stress resulting from the fatigue pre-crack formation step, and the sensor mounting step of mounting the sensor body on the structural material with the mean stress of the sensor body increased.
In the sensor mounting step, the detection sensor body, which has been heated, can be mounted on the structural material.
This pre-heating operation causes the detection sensor body to be contracted when cooled to increase its mean stress due to a difference in temperature and thermal expansion between the sensor body and the structural material.
An appropriate difference in temperature between the detection sensor body and the structural material caused by pre-heating has been found to be about 10xc2x0 C. or higher, preferably about 30xc2x0 C.
In the sensor mounting step, the sensor body can be linearly heated while being mounted on the structural material.
This linear heating can also increase the mean stress as in the preheating operation.
Means for mounting the detection sensor body on the structural material may be arbitrary and include adhesion, welding, and bolting, but an adhesion means is preferable due to the easiness of this operation.
Although a step is required that mounts the crack detection means on the detection sensor body in front of the tip of the notched portion, this step may be executed before or after machining the notched portion in the sensor body.
The crack detection means must only be able to detect a crack and may include a crack gauge, an optical crack detection apparatus, or a crack detection apparatus using ultrasonic waves.
In a fatigue damage detection sensor for structural materials and mounting method thereof according to this invention, the detection sensor body with the notched portion formed therein and the crack detection means can be used to detect fatigue damage caused to the structural material with a predetermined sensitivity or detection accuracy, and the sensor can be mounted so as to execute sensing with a stable sensitivity.
In particular, based on the proportionality of the crack propagation rate da/dN to H1.5, the fatigue damage detection sensor according to this first invention can adjust the crack propagation rate da/dN to control a required sensitivity. Thus, by setting the lateral length (in a strain (displacement) constraining method for fixing both ends of the sensor body to the surface of the structural material, the length between the pair of fixing portions) of the sensor body at a predetermined value, the sensor body can be mounted with a desired sensitivity.
In the method for mounting a fatigue damage detection sensor according to this second invention, the fatigue pre-crack is formed in the notched portion of the sensor body beforehand, a residual stress generated during the formation of the fatigue pre-crack is relieved, and the mean stress is increased before mounting the sensor body. These apparatuses can ensure that a crack stably occurs and propagates without being affected by external environments such as the temperature and humidity, thereby enabling the history of stress or accumulated fatigue of the structural material to be sensed with practical construction costs and workability using the sensor with the notched portion formed therein.
Even if an actual stress caused by a working load is small, the increased mean stress allows a crack to occur and propagate, and the propagation rate of a crack linearly varies relative to the stress intensity factor range xcex94K proportional to H0.5, thereby enabling sensing with a predetermined accuracy.
A desirable mean stress corresponds to a tensile stress of stress ratio 0.6 or more. In addition, by allowing the tensile stress to remain, sensing is possible even in a compressed field of the structural material.
In this way, when a working load acting on a structural material of a bridge or the like causes a crack to propagate up to a predetermined threshold, this can be visually confirmed or electrically detected using a crack gauge or a crack detection gauge or apparatus for an alarm or further detailed inspections.
That is, when simply affixed to a structural material, the fatigue damage detection sensor according to this invention can detect the history of stress (history of the magnitude of stress and of the number of stress cycles) acting on the structural material and record the accumulated history of stress so that these records can be used to determine the stress or fatigue accumulated during an arbitrary period in order to predict fatigue damage that may be caused to the structural material.
Next, a fatigue damage detection sensor 1 for structural materials and mounting method thereof according to an embodiment of this invention will be described with reference to FIGS. 1 to 9.
FIG. 1 is a top and front views of the fatigue damage detection sensor comprising an indicator plate 2 (a sensor body), a crack gauge 3 that is an example of a crack detection means, and a lateral pair of affixed portions 4 (fixing portions). In the figure, an example of the size of each part is described in the unit of mm.
The indicator plate 2 constitutes the body of the fatigue damage detection sensor 1, is shaped like a rectangle having a specified width and a predetermined length, and has a generally long V-shaped notched portion 5 on one side.
The crack gauge 3 is placed so as to be opposed to a tip 5A of the notched portion 5 so that stress is concentrated at the tip 5A to allow a crack to occur in the indicator 2 sufficiently earlier than in a base material M when loaded.
The notched portion 5 need not be formed at the center of the indicator plate 2. It may be formed on both sides of the indicator plate 2 or inside the plate.
The crack gauge 3 preferably determines the length of a crack in the indicator plate 2 more easily when placed perpendicularly to the propagation direction of a crack propagating from the tip 5A, and may be a strain gauge 6 or a parallel arrangement of electric resistance wires. A fatigue crack is formed at an end of the tip 5A beforehand, as described below in FIGS. 2 and 3.
The lateral pair of affixed portions 4 are used to fixedly mount the fatigue damage detection sensor 1 on the base material M over a predetermined area. The fatigue damage detection sensor 1 and the base material M are integrated together at the affixed portions 4, and the length of part of the fatigue damage detection sensor 1 located between the affixed portions 4 independently of the base material M is defined as 2H.
The material of the indicator plate 2 and an adhesion for the affixed portions 4 affixed to the base material M may be arbitrary but desirably have durability and a weather resistance and allow the correlationship with stress acting on the base material M and the crack gauge 3 to be stably determined. In general, the adhesion can be thermally set and the indicator plate 2 is formed as thin as possible.
For example, the indicator plate 2 has a thickness of 1 mm or less and a width varying depending on the length of the sensor. If 2H is set at 100 to 200 mm, the width is 10 to 100 mm.
The length of the indicator plate 2 is determined based on a sensitivity required of the damage fatigue sensor 1.
As shown in FIG. 1, when the length of a crack C that can propagate from the tip 5A of the notched portion 5 is defined as (a), the number of times that a working load acts on the material, and the crack propagation rate is defined as da/dN, da/dN is proportional to H1.5. Thus, the length 2H between the affixed portions 4 is set so as to obtain a predetermined sensitivity.
That is, when both ends (affixed portions 4) of the fatigue damage detection sensor 1 are fixed and a specified strain xcex5 is applied to these ends and if the modulus of longitudinal elasticity is defined as E, the stress intensity factor K of the tip of the crack C is expressed as follows:
K=Exc2x7xcex5xc2x7H0.5
When the fatigue damage detection sensor 1 is affixed to the base material M having a strain variation range xcex94K, the stress intensity factor range xcex94K is expressed as follows:
xcex94K=Exc2x7xcex94xcex5xc2x7H0.5
The crack propagation rate da/dN is proportional to about the third power of xcex94K, so it is proportional to H1.5.
FIG. 2 is a flowchart showing a procedure for mounting the fatigue damage detection sensor 1. First, the notched portion is machined (step S1).
No fatigue crack occurs in the machined notched portion 5 under such a low stress as generated in the base material M that is an actual structural material or a long time period is required before a crack occurs in this portion.
Thus, a fatigue pre-crack 7 (see the enlarged view in FIG. 3) is formed at step S2.
FIG. 4 is a schematic explanatory drawing of a fatigue pre-crack generator 8 for forming the fatigue pre-crack. The fatigue pre-crack generator 8 comprises a frame 9, a pair of fatigue damage detection sensor grabbing portions 10, and a fatigue pre-crack monitoring sensor 11 wherein the fatigue damage detection sensor grabbing portions 10 are vibrated at predetermined cycles to generate the fatigue pre-crack 7 in the notched portion 5 of the fatigue damage detection sensor 1.
Since the stress used to generate the fatigue pre-crack 7 is set higher than a stress normally occurring in the structural material, the crack C may not propagate stably due to a residual stress generated at the tip of the fatigue pre-crack 7.
Thus, the fatigue damage detection sensor 1 with the fatigue pre-crack 7 formed therein is annealed to relieve the residual stress at step S3 (FIG. 2).
FIG. 5 is a schematic explanatory drawing describing this annealing operation in brief. The fatigue damage detection sensor is accommodated in a heating furnace 12 and heated therein at a predetermined temperature for a predetermined time period. The fatigue damage detection sensor 1 is then left and the furnace is cooled for annealing to relieve the residual stress generated due to the formation of the fatigue pre-crack 7.
Returning to FIG. 2, the fatigue damage detection sensor 1 with its mean stress increased is mounted on the base material M at step S4.
That is, since a repeated stress that may occur in a normal base material M is not so high, the fatigue crack C may not propagate stably in the fatigue damage detection sensor 1. The fatigue stress C has been confirmed to propagate stably even under s low stress if the mean stress of the fatigue damage detection sensor 1 is increased in advance. The mean stress must be at least 30 MPa.
This mean stress can be applied by, for example, applying an initial stress. The fatigue damage detection sensor 1 can be preheated before mounting on the base material M by means of adhesion, welding, or other mechanical fixing means such as bolting.
Alternatively, after the fatigue damage detection sensor 1 is mounted on the base material M by adhesion, welding, or bolting, the portion of the sensor corresponding to the indicator plate 2 is linearly heated and then cooled to allow the indicator plate 2 of the fatigue damage detection sensor 1 to be contracted in order to apply the initial stress.
FIG. 6 is a schematic explanatory drawing showing this linear heating operation. For example, after the fatigue damage detection sensor 1 has been fixed to the base material M by welding fixing portions 13, the portion of the fatigue damage detection sensor 1 corresponding to the indicator plate 2 is heated in the width-wise direction (a linearly heated portion 14) and then cooled and contracted to generate an initial stress therein in order to increase the mean stress.
In addition, FIG. 7 is a schematic explanatory drawing showing the bolting operation. Bolts 16 are inserted into the indicator plate 2 in a lateral pair of bolting areas 15 (fixing portions) to fix the fatigue damage detection sensor 1 to the base material M. Long holes 17, however, are formed in one of the bolting areas to enable the level of the mean stress to be adjusted based on their positions relative to the bolts 16.
FIG. 8 is a graph showing the stress intensity factor range xcex94K vs. the crack propagation rate da/dN in cases where an initial stress is and is not applied to the indicator plate 2. This graph shows that the application of the initial stress increases the mean stress to provide a linear property and allows the crack C to occur and propagate even under a small stress, whereas no crack C occurs under a small stress if the initial stress is not applied.
In this way, this indicates that a predetermined sensing accuracy and sensitivity (the stress intensity factor range xcex94K and the gap propagation rate da/dN) can be obtained by setting the length H in the indicator plate 2 at an arbitrary value.
FIG. 9 is a graph showing a preheating operation for the fatigue damage detection sensor 1 and temporal changes in the temperature of the sensor, showing how the temperature of each part of the fatigue damage detection sensor 1 increases.
This preheating method employs, for example, an H-shaped steel as the base material M, adheres the fatigue damage detection sensor 1 to the center of an upper flange 18 of the H-shaped steel, places magnets 19 on the right and left to the sensor 1 to press the affixed portions 4 against the upper flange 18 with a predetermined pressure, places a preheating heater 20 between the magnets 19, and heats the center of the indicator plate 2, as shown in the figure in the graph.
The graph shows that the difference in temperature between the center of the fatigue damage detection sensor 1 and the base material M (the center of the upper flange 18) becomes almost constant (about 30xc2x0 C.) after a predetermined time period.
By fixing the fatigue damage detection sensor 1 to the base material M while maintaining this constant temperature difference, the fatigue damage detection sensor 1 allows the crack C to be stably generated and propagated and detects it despite a difference from the temperature of the external environment (for example, xe2x88x9220xc2x0 C. to +50xc2x0 C.). Consequently, the sensor 1 can sense the level of fatigue damage caused to the base material M.